Gear grinding machines are configured to sequentially grind the tooth surfaces of a gear mounted on a rotary table by rotating the gear and inserting the circumference of a disk-shaped grinding wheel, which is being driven to rotate, into each tooth space of the gear. In this way, the gear's meshing accuracy can be improved. Such a gear grinding machine cannot properly grind the tooth surfaces of the gear when the axis of the gear is displaced (offset) with respect to the axis of the rotary table. Thus, the axis of the rotary table and the axis of the gear need to be accurately positioned to each other (centering). However, in a case of machining a large-sized gear measuring several meters in diameter, the centering cannot be done easily because such a gear is extremely heavy, weighing several tons.
In this respect, for example, Patent Literature 1 listed below and the like disclose the following. Specifically, a rotary table is rotated to measure the runout of a gear to thereby read, from a graph, the relation between the value of the position of the axis of the gear (eccentricity) and the phase position. As position vectors associated with the center of the rotary table, the following are calculated: the eccentricity at the center in the face width; the crossing angle between the axis of the rotary table and the axis of the gear; the eccentricity with respect to any planes perpendicular to the axis of the rotary table; and the phase angle. Then, the above adjustment data thus obtained are displaced perpendicular to the axis of the rotary table so that the axis of the rotary table and the axis of the gear can intersect at the center in the face width, for example. Then, the axis of the rotary table and the axis of the gear are tilted by the crossing angle between these two axes, followed by the moving of a tool in accordance with such speeds, positions, and paths that the guiding axis of the tool and the axis of the gear coincide, to thereby machine the gear.